1. Technical Field of the Invention
The present invention relates to the use of errors and erasures decoding, and more particularly, to a method for determining the bits to be erased in an errors and erasures decoding system.
2. Description of Related Art
When digital data is wirelessly transmitted over physical channels, the physical channels may corrupt the transmitted digital waveform with additive noise and multipath fading resulting in occasional errors at the receiver. Reliable communications over fading channels requires a large bit energy to noise ratio. It is known that when communicating over a Rayleigh fading channel, the uncoded bit error rate (BER) decreases inverse linearly rather than exponentially with information bit energy to noise ratio E.sub.b /N.sub.o, both being plotted on a logarithmic scale. Fading can also cause a loss in capacity and reduce channel cutoff rate.
To compensate for the detrimental affects of channel fading and additive white gaussian noise, most communications systems use some form of error correction coding. Most of the losses incurred from channel fading can be recovered using error control coding with some optimally-selected coding rate. Coding involves the adding of redundancy to the transmitted sequence to enable the receiver to correct for errors.
A method for protecting against channel fading and noise using error correction decoding is the use of errors and erasures decoding. In a TDMA communications system, users are assigned timeslots containing a number of data symbols for transmission. If the system uses some form of error correction coding, it is desirable to obtain information concerning the reliability of the symbols in a particular timeslot, erase unreliable symbols and use errors and erasures correction decoding to decode the information. The error correcting capability of channel codes is determined by two factors; the code weight (or distance) distribution of the code and the decoding algorithm. For a code with a minimum Hamming distance d, the decoder can correct all error patterns if the number of errors e satisfies the inequality 2e.ltoreq.d-1. However, the code can also correct all error patterns and erasure patterns if the number of errors e and erasures .gamma. satisfy the inequality 2e+.gamma..ltoreq.d-1.
In a hypothetical limit wherein all errors occurring within the received message could be identified and erased before decoding (i.e., e becomes 0), the erasure correcting capability of a particular code using erasures decoding mode would be effectively doubled over the error correcting capability of the same code using error decoding. In practice, not all errors within a received message can be identified with certainty. Thus, it is highly desirable to develop erasure techniques for indicating whether a demodulated symbol is reliable, enabling erasure of the unreliable symbol prior to decoding. The decoder is then able to correct twice as many identified erasures as random, unidentified errors.
Additional techniques for obtaining reliability information about a channel having coded communications generally include pre-detection techniques and post-detection techniques. Pre-detection techniques attempt to determine, for example, signal to noise or interference ratio, but this is difficult in an environment where the noise and interference fade up and down as well as the signal. In a frequency hopping system the technique has been used to generate reliability information concerning a particular hop or burst by deeming a hop to contain interference if its signal strength is unusually high. This illustrates that the interpretation of high signal strength as high quality is not always valid. All post-detection methods for erasures described in the prior art involve making hard decisions on test bits, thus resulting in a loss of performance, as "hard" decisions are known to give poorer decoding than "soft" decisions.